The present invention relates to the use of palladium-silver alloys for the manufacture of fixed or removable dentures, which can be veneered with dental ceramics.
Fixed or removable dentures are manufactured primarily from corrosion-resistant, biocompatible precious-metal alloys with the so-called precision investment casting, also referred to as the lost-wax process in which the cast object is frequently veneered with dental ceramics in order to achieve an appearance which corresponds to the natural tooth. To be compatible with the dental ceramics the alloys must exhibit special properties, such as coefficient of thermal expansion, melting range or adhesion.
Alloys with a high gold content such as those described, for example, in German patents 11 83 247 and 15 33 233 are especially well-suited for this purpose. However, due to the high and very fluctuating price of gold, more of an effort has been made recently to find more economic alternatives to the alloys with a high gold content. Palladium presents itself from the group of noble metals as an appropriate substitute on account of its relatively economic price, its density, which is considerably less in comparison to that of gold, and its resistance to corrosion in the mouth (oral environment), which is comparable to that of gold.
It is to make a distinction between alloys containing silver and those not containing silver in the previously known palladium-based alloys in the field of dentistry.
Silver-free palladium-based alloys contain copper, tin, indium, cobalt and gallium as main alloying elements. Typical silver-free palladium-based alloys are described for example in German patents 33 16 595; 33 04 183; 33 14 657 or 35 22 523. Compared with alloys with a high gold content, these alloys react more sensitively to manufacturing errors and are difficult to solder. They absorb considerable amounts of carbon in the liquid state, so that they should only be melted in a ceramic crucible. The dark oxides which form during the fixing of the dental ceramics at approximately 980.degree. C. adversely affect the aesthetic appearance of the denture by the formation of dark margins in the edge area of the veneering.
Silver-containing palladium-based alloys fall between the alloys with a high gold content and the silver-free palladium-based alloys as concerns their working behavior. As a result of the silver portion, they are easier to melt and to cast, have a brighter oxide and a good soldering behavior. In addition,they are more economical than the silver-free palladium-based alloys.
The typical composition of such alloys can be found in the "Survey of Dental Noble-Metal Alloys and Dental Base Metal Alloys in the Federal Republic of Germany" [in German] published by the Research Institute for Dental Care (FZV), Vol 1., (July 1986), pp. 31-2. In addition to palladium and silver, these alloys contain primarily tin, indium and zinc, in individual instances also copper or gallium as further alloying elements.
The disadvantage of these alloys is the fact that they discolor the dental ceramic to a yellow or a yellowish green during the firing process. This discoloration is caused by the silver, which passes by diffusion or via the vapor phase into the ceramics.
DE-PS 25 23 971 describes palladium-silver alloys which contain 0.1 to 0.5 % titanium in order to suppress the discoloration of the ceramics. As a result of the reactivity of titanium with the atmospheric oxygen and/or the crucible materials, the melt becomes impoverished of this element relatively rapidly so that the reducing action on the tendency toward discoloration is lost when using old material (runners, casting funnels) and under unfavorably selected melting conditions. Moreover, the titanium brings about a strong adhesion of the investment material to the surface of the cast object, which makes it more difficult and more time-consuming to divest and to finish it.
U.S. Pat. No. 4,350,526 describes palladium-silver alloys which exhibit no discoloring action on dental ceramics due to the addition of 0.1-1.0% silicon. Silicon is insoluble both in palladium and in silver. Moreover, palladium and silicon form intermetallic phases, so that a strong embrittlement of the alloy and fracture after casting can occur.
Silicon favors, similar to titanium, a reaction with ceramic materials, so that a strong adhesion of the material to the cast object also occurs in the case of these alloys.